Distance from a known object to a target object can be measured in many non-contacting ways. Electromagnetic energy is used for RADAR data, acoustical energy is used for SONAR data, and light is used for LIDAR data. RADAR uses radio waves, SONAR uses acoustical waves, and LIDAR uses light waves. These three distance determining techniques rely on time-of-flight measurements to determine distance. Other techniques include the use of structured light, interferometry, and various vision systems. Also, conventional force and pressure sensors can be used. Another technique uses the measurement of back-pressure from a gas jet impinging on the surface of the object being sensed.
One structured-light method for determining distance between two non-contacting objects, i.e., proximity between two objects, is described in U.S. Pat. No. 4,479,053, entitled "Focal Plane Array Optical Proximity Sensor," to Johnston. An optical system mounted in a first object senses the relative position of a surface of a second object. This optical proximity sensor works by utilizing the intersection of an illuminated light field with a light-detecting field-of-view to define a detection volume. Light reflected from the surface of an object in the detection volume is sensed by a photodiode and indicates that an object is within the volume defined by the intersecting fields. The light emitters in Johnston are light-emitting diodes, and the light detectors are photodiodes. There is a photodiode for each light-emitting diode, and each photodiode is located on the image plane of a lens whose conjugate object plane intersects the aforementioned intersection volume. By appropriately positioning each light-emitting diode/photodiode pair on its respective focal plane, and appropriately positioning the lens assemblies with respect to each other, any desired volume can be defined in which an object is to be detected.
Another method using structured light for determining the distance between two non-contacting objects is described in U.S. Pat. No. 4,687,325, entitled "Three Dimensional Range Camera," to Corby, Jr. A pattern generator and projector on the first object produce a 1 N array of time/space coded light rays whose intensities can be varied with time, and projects P sequential presentations of different subsets of the light rays onto the second object, where P=1+logbN, where b is the number of brightness levels, and where N is the number of rays. The light rays are projected onto the second object along a direction which is not coaxial with the optical axis of an imaging device. A linear sensor such as a line scan camera images the points of light where rays are incident on the surface of the second object and generates one-dimensional scan signals which have ray-associated peaks at locations corresponding to imaged light. A high speed range processor analyzes the one-dimensional waveforms to uniquely identify all rays, determines depth from the displacement of the ray peaks from their zero-height reference-plane peaks, and provides output range data. To create two-dimensional range maps, a means such as a rotating mirror is provided for orthogonally sweeping the 1N coded light rays by steps over a rectangular plane.
While these structured-light methods work well for their intended purposes, they are not able to sense contact with another object. Force and pressure sensors however can be used to measure contact between two objects, but cannot sense proximity between the two objects unless there is actual contact. As well, proximity sensors and contact sensors are not necessarily well suited to sense lateral displacement between the sensor on the first object and the target surface on the second object when the sensor-to-target separation is small.
What is needed is a method and system for sensing close proximity of one object to another, touch or contact between two objects, and lateral movement or "slip" between two objects in contact or nearly in contact with one-another.